The field of the invention relates to control of direct injection engines. In particular, the field relates to control of air/fuel mode transitions for direct injection spark ignition engines.
In conventional port injected engines, which induct a mixture of air and atomized fuel into the combustion chambers, control systems are known which adjust engine torque by controlling the air throttle. It is also known to control engine torque by advancing or retarding ignition timing. An example of such a system is disclosed in U.S. Pat. No. 5,203,300.
The inventors herein have recognized numerous problems when applying known engine torque control systems to direct injection spark ignition engines in which the combustion chambers contain stratified layers of different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. Use of conventional torque control systems for this type of engine is recognized by the inventors herein to be inadequate because stratified operation is unthrottled so the throttle is not a viable control variable. And ignition timing is not a viable control variable because the timing must be slaved to the time a rich air/fuel strata is formed near the spark plug. These problems are further exasperated in direct injection spark ignition engines which have two modes of operation--the stratified mode discussed above and a homogeneous mode in which a homogeneous air/fuel mixture is formed at the time of spark ignition.
A particular problem in controlling engine torque in a DISI engine is transitioning between one mode of operation to the other while maintaining a controlled engine torque. This is necessary to prevent sudden dips or bumps in engine speed caused by a sudden drop or rise in engine torque. For example, this is important during the idling operation where a mode transition from stratified to homogeneous is necessary to purge fuel vapors in the vapor recovery system.